Histone H4 is one of the four histones, along with H2A, H2B and H3, which forms the eukaryotic nucleosome core. Along with H3, it plays a central role in nucleosome formation. The sequence of histone H4 has remained almost invariant in more then 2 billion years of evolution [<cite idref="PUB00004448"/>, <cite idref="PUB00003642"/>, <cite idref="PUB00004015"/>].